Check double variable if it contains an integer, and not floating point

Assuming a c99 and IEEE-754 compliant environment,

(trunc(x) == x)

is another solution, and will (on most platforms) have slightly better performance than modf because it needs only to produce the integer part. Both are completely acceptable.

Note that trunc produces a double-precision result, so you don't need to worry about out of range type conversions as you would with (int)x.

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Edit: as @pavon points out in a comment, you may need to add another check, depending on whether or not you care about infinity, and what result you want to get if x is infinite.

Use std::modf:

double intpart;
modf(value, &intpart) == 0.0

Don't convert to int! The number 1.0e+300 is an integer too you know.

Edit: As Pete Kirkham points out, passing 0 as the second argument is not guaranteed by the standard to work, requiring the use of a dummy variable and, unfortunately, making the code a lot less elegant.